The Air Car Preps for Market

Some still question the vehicle's chances of success, despite a boost from India.

Air car: MDI says that its MiniCat compressed-air car (above) can reach speeds of up to 100 miles per hour and has a range of up to 125 miles. Credit: Motor Development International

A French-designed car that's propelled by compressed air and claims speeds of more than 60 miles per hour is expected to go into commercial production as early as this summer, although skeptics of the technology aren't holding their breath.

Using compressed air, they argue, may mean zero tailpipe emissions, but it's unlikely to provide enough range or speed to appeal to the masses, particularly in North America. "Compressed air does not contain much energy--that's the killer," says Larry Rinek, senior research analyst for automotive technologies at consultancy Frost & Sullivan. "This is more a nice garage project for a Popular Science subscriber."

But the dream lives on. Motor Development International (MDI), based near Nice, France, has developed several prototypes of its Compressed Air Technology (CAT) car since its first engine was created 14 years ago. Now company founder Guy Negre, an aeronautics engineer who developed a high-performance racing engine for Formula 1 in the late 1980s, is counting on India's largest carmaker, Tata Motors, to bring his highly anticipated Air Car to market later this year.

The Air Car was supposed to hit the streets years ago, but its release always seems just around the corner. MDI announced in 2002 that the cars would be used to replace taxis in Mexico City, but nothing resulted.

Tata's involvement this time around, combined with the fact that oil recently hit $100 a barrel, could change the game. India's largest automaker announced last February that it had struck a deal with MDI to further develop and refine Negre's compressed-air engine technology, with the intention of producing and selling the emission-free cars in India. It has since been reported that Tata invested nearly $30 million in MDI as part of the agreement.

"The recent manufacturing push is in response to the contract that MDI signed with Tata," confirmed Kevin Haydon, a spokesman for Zero Pollution Motors, based in New Paltz, NY. He says that the company plans to manufacture CAT vehicles in parts of the United States around 2010, through a license with MDI.

Zero Pollution has even entered the car in the multicity Automotive X Prize competition, where in 2009 more than 30 teams--including electric carmakers Tesla Motors, Phoenix Motorcars, and Malcolm Bricklin's Visionary Vehicles--will compete on the fuel efficiency of their vehicle designs.

The Air Car may do better than fuel-cell cars, but experts say that using grid power to charge a battery-powered electric vehicle is much more efficient than using electricity to compress and store the same amount of energy in a tank. "The main problem is that air gets hot when you compress it, so much of the energy input goes into raising the temperature of the air as you try to raise the pressure," explains Doug Nelson, a professor of mechanical engineering and an expert on advanced vehicle systems at Virginia Polytechnic Institute.

According to its website, MDI insists that, compared with the most prevalent cars on the road--those powered by gasoline--its air engine is "far superior in terms of energy used and thermodynamics."

At the core of all CAT models is a four-piston engine powered by compressed air that is stored in tanks at 4,500 pounds per square inch. The lightweight tanks, a thermoplastic container surrounded by a carbon-fiber shell, are made by Airbus Industries and hold nearly 3,200 cubic feet of air.

To propel the vehicle, compressed air from the tanks is injected into a small chamber, where it expands and cools. This expansion drives a downstroke of the piston. But as the ambient temperature begins to reheat the air in the first chamber, that air is forced into a second neighboring chamber, where it expands again to drive an upstroke. Using ambient heat helps capture more of the energy in the compressed air, ultimately improving the efficiency and expanding the range of MDI's Air Car. And compared with four-stroke combustion engines, in which half of the strokes are wasted to pull air and fuel into the chamber, the air engine makes use of every stroke.

Ulf Bossel, a mechanical engineer consulting in Switzerland and organizer of the European Fuel Cell Forum, is cautiously optimistic. "I think there's something to it," says Bossel, one of the few who has performed a comprehensive analysis of MDI's approach. Even though one of MDI's compressed-air tanks would carry the energy equivalent of just one gallon of gasoline, the use of that air in the engine is 90 percent efficient.

The energy balance would improve substantially, he argues, if the compressed-air systems located at filling stations or in car owners' garages were designed so that any waste heat during compression could be captured and used to produce domestic hot water, for example. If the compressors could interact with the grid and be programmed to only compress and store air during off-peak hours, or when solar and wind energy are in greater supply, the emissions profile of the Air Car would also improve.

"If you use clean electricity, it's an absolutely clean system," says Bossel, adding that compressed-air systems, despite being less efficient than battery-powered cars, have the advantage of being simple, cheaper to manufacture, and unconstrained by the degradation problems associated with current battery systems. "Still, there are some thermodynamic tricks you have to do," he says.

To increase the range of the vehicle, MDI is also coming out with an optional dual-mode system that allows the car to run on fossil or biofuels--either when its speed exceeds 35 miles per hour or when the compressed-air tanks are empty. When in fuel mode, the car's moto-alternator refills the tank with compressed air as the vehicle moves.

MDI says that in air-only, zero-emission mode in urban settings, the car has a range of up to 125 miles, but that drops to about 50 miles when it is driven at faster speeds up to 60 miles per hour. A full charge of air, including the electricity used to compress it, is expected to cost less than $3. In the dual-mode version, with assistance from fuel, speeds can reach 100 miles per hour, and range expands to 900 miles on less than a gallon of fuel (although the faster one goes, the shorter the range).

The idea is for the car to provide zero pollution when driven in cities, and lower emissions when driven faster, as in suburban and rural settings. "It's a little more like a hybrid," says Haydon. "The new innovation will make it more appealing to the average buyer. In terms of the carbon dioxide emissions, it will have five times less than the average vehicle."

The first CAT car to be produced is called the OneCAT, a "utilitarian" car for urban and rural driving that's specifically designed for use in overcongested cities and priced in a range ($5,100 to $7,800) within reach of consumers in a developing economy, such as India.

The ultralight bodies of the vehicles would be made of glued-together fiberglass and injected foam, and the aluminum chassis would also be glued, not welded, to simplify manufacturing.

Before production of the CAT can begin in India, likely followed later by Spain and Australia, MDI says that it must start mass production of the cars at its factory in France. A spokeswoman at MDI's marketing office in Barcelona says that this will likely happen in September 2008.

But Frost & Sullivan's Rinek believes that there's limited chance that major automakers in the United States will take the Air Car seriously. He points to the need to build an infrastructure of compressor stations and the need to comply with strict safety standards. (The Air Car cannot be fueled using the air pumps currently found at some gas stations.)

"In North America, it's basically a nonstarter," says Rinek, admitting that there are limited niche markets. "The only potential, if any, would be for an inner-city, short-commute vehicle with an ultra-greenie owner."

http://www.technologyreview.com/Energy/20071/